1. Field of the Invention
The present invention relates to an accumulator for use in controlling an in-riser or open water intervention system such as a subsea stack of Subsea Test Tree (“SSTT”) and the valves associated therewith.
2. Description of the Prior Art
Accumulators are devices that provide a reserve of hydraulic fluid under pressure and are used in conventional hydraulically-driven systems where hydraulic fluid under pressure operates a piece of equipment or a device. The hydraulic fluid is pressurized by a pump that maintains the high pressure required.
If the piece of equipment or the device is located a considerable distance from the pump, a significant pressure drop can occur in the hydraulic conduit or pipe which is conveying the fluid from the pump to operate the device. Therefore, the flow may be such that the pressure level at the device is below the pressure required to operate the device. Consequently, operation may be delayed until such a time as the pressure can build up with the fluid being pumped through the hydraulic line. This result occurs, for example, with deep water applications, such as with SSTT and BOP equipment, which is used to shut off a well bore to secure an oil or gas well from accidental discharges to the environment. Thus, accumulators may be used to provide a reserve source of pressurized hydraulic fluid for this type of equipment. In addition, if the pump is not operating, accumulators can be used to provide a reserve source of pressurized hydraulic fluid to enable the operation of a piece of equipment or device.
Accumulators conventionally include a compressible fluid, e.g., gas, nitrogen, helium, air, etc., on one side of a separating mechanism, and a non-compressible fluid (hydraulic fluid) on the other side. When the hydraulic system pressure drops below the precharged pressure of the gas side, the separating mechanism will move in the direction of the hydraulic side displacing stored hydraulic fluid into the piece of equipment or the device as required.
When a conventional accumulator is exposed to hydrostatic pressure, such as encountered in subsea operations, the available hydraulic fluid is decreased since the hydrostatic pressure must first be overcome in order to displace the hydraulic fluid from the accumulator. Once the conventional accumulator begins to displace fluid, the pressure of the non-compressible fluid decreases and cannot overcome the hydrostatic pressure thus causing the remaining fluid in the conventional accumulator to become essentially unusable. This is typically compensated for by increasing the precharge in the secondary chamber in the conventional accumulator to compensate for the hydrostatic pressure. In these conventional accumulators, the precharge must usually be adjusted for each operating depth in order optimizes the conventional accumulators' available liquid volume. In a deep subsea well, the gas precharge pressure may be higher than the hydraulic fluid pressure rendering the accumulator useless when testing the hydraulic circuit at the surface. A conventional accumulator has the further shortcoming that it cannot be used at several different depths and unless it is used at the depth for which it is configured, it may still have an amount of unusable hydraulic fluid.
Pressure-balanced accumulators have been proposed to overcome the above-described shortcomings of a conventional accumulator. Pressure-balanced accumulators are, for example, disclosed in U.S. Pat. No. 6,202,753 to Benton and U.S. Patent Publication No. 2005/0155658-A1 to White.